Narcotic analgesics (i.e. opiates such as morphine) and endogenous opioid peptides bind to the three types of opioid receptors (mu, delta, kappa) with varying affinities and activate these receptors to different extents. Opioid receptor function is modulated by multiple mechanisms; the long-term objective of the project is to understand the contribution of these mechanisms to receptor activity. This application is focused on exploring a recently recognized mechanism of 'receptor dimerization/oligomerization' in modulating opioid receptor function. We and others have previously shown that mu receptors dimerize with delta receptors and this changes receptor properties. Recently we generated mu-delta heterodimerselective antibodies. Using these we have found that levels of mu-delta heterodimers can be differentially regulated by opiates in vivo. The functional consequences of this regulation and the molecular mechanisms involved are not known. In this application we propose studies to directly address this. Specific Aim #1 is to examine the novel signaling pathways activated by the mu-delta heterodimer using classic biochemical techniques. Aim # 2 is to investigate the distinct maturation/assembly and trafficking properties of the heterodimer using cell biological and imaging techniques. Aim # 3 is to study the distribution and regulation of the heterodimer by morphine in vivo using immunohisto-chemical techniques. The studies described in this application will characterize the role of mu-delta heterodimers in modulating opiate signaling, and explore the physiological significance of mu-delta heterodimerization in vivo. For this we propose to use a combination of classical techniques with novel tools and state-of-the art technology. These studies will help characterize the mechanism of receptor maturation/upregulation and delineate distinct pathways activated by the heterodimer. The identification of molecules and pathways unique to opioid receptor heterodimers is likely to open new avenues and/or targets for intervention in drug abuse and addiction.